The ship made its first transatlantic voyage in early May, when it sailed from Southampton in the UK to New York, and has since been offering private guests - including travel agents - and members of the public a glimpse on board.

But on Sunday evening, it set sail from Miami, Florida, with 3,600 paying guests on board on the first of its commercial seven-day trips to the Caribbean.

Passengers can surf using the ship's wave machine

A floating city

The vessel boasts several pools, an open-air nightclub, whirlpools cantilevered off the side of the ship, a full-size boxing ring and a Royal Promenade "entertainment boulevard".

Although the ship is not as long or as high as the QM2, which remains the world's biggest ocean liner, it is bigger by volume.

A review on the Frommers travel website gave the ship a "B+", saying the Royal Promenade was "dominated by shops and corporate co-branding arrangements... giving it a feel that's as much mall as theme park", but "does offer that big, active, city-vacation feel".

However, luxury comes at a price.

Passengers looking to take a week-long cruise during the summer months will pay between $1,500 (£800) and $3,200 (£1,700) for a ticket, spokeswoman Tracey Quan told the BBC news website.

The company had already taken multiple bookings and there was only limited availability between now and December, she said.

Over the next two years, Royal Caribbean is planning to add two other similar-sized ships to its luxury fleet.

Liberty of the Seas will take its maiden voyage in April 2007 and a second vessel, currently named Freedom III, will be introduced in April 2008.

'Project Genesis'

Royal Caribbean is also working on a much larger project which will not see the light of day until 2009.

The new liner, code-named Project Genesis, will dwarf the Freedom of the Seas, measuring 220,000 tons and will have the capacity for up to 5,400 passengers.

Tricia Barnett of Tourism Concern said there were ethical concerns with travelling on cruise liners. The ships leave behind waste and pollution, she said, but rarely pick up local produce and goods.

She also said previous studies of cruise shops highlighted problems of staff being paid low wages and enduring bad conditions.

Royal Caribbean refused to comment on the pay-scale or working conditions for the luxury liner's 1,360 crew.

Asked about environmental policies aboard the ship, which has a fuel capacity of 3,533 tons, the spokeswoman confirmed that all provisions were purchased in Miami and the vessel did not restock along the way.